Implantable medical devices may be implanted into patients for a variety of reasons, including, for example, to improve the clinical condition of a patient, to replace natural patient tissue, or for aesthetic purposes. In many cases, implantable medical devices are implanted in patients having severe, complex, or chronic medical conditions. For example, breast implants may be used in reconstructive surgeries following mastectomies, e.g., after a cancer diagnosis, surgical removal of breast tissue, radiation therapy, and/or chemotherapy.
There are many situations in which implantable medical devices and the tissue in which they are implanted may need to be examined, monitored, identified, or further altered after implantation, either by invasive or noninvasive means. For example, after implantation of a medical device, follow-up may be required to monitor healing, check for clinical improvement, and/or screen for development or reappearance of other medical conditions in the vicinity of the medical device (e.g., the reappearance of cancerous tissue in a patient in remission). As a further example, it may be advantageous to be able to identify characteristics of an implanted device, such as the device's model, size, shape, lot number, or other characteristics, without performing an invasive procedure to visually inspect the device. As yet another example, some implantable medical devices may require adjustment after implantation. For example, tissue expanders, such as those which may be used in patients undergoing breast augmentation or reconstruction surgery, may be designed to be incrementally expanded over time.
Various technologies have been developed in order to improve the safety and efficacy of breast implants and other implantable medical devices, in part to address some of the above concerns. Among these technologies is the use and integration of transponders, such as radio-frequency identification (RFID) transponders, in implantable medical devices. Such transponders may be used, for example, to transmit information from within a patient's body, such as information about a location of the device in the patient's body, or a location of a portion of the device in the patient's body. As another example, such transponders may be used to transmit information about an implanted device itself by way of, e.g., a serial number encoded on a chip in each transponder. Information about the implanted device may be useful for, e.g., determining whether the device is subject to any recalls, determining the materials in the device, and planning further surgeries. Information about implantable medical devices may also be useful prior to implantation, such as to track the devices from manufacturing, through storage, sale, transport, delivery to medical centers, and implantation in patients. Microtransponders, such as transponders which have a length of less than three centimeters and a width of less than a centimeter, may provide the added advantage of being small enough for inclusion within implantable medical devices without substantially affecting, e.g., the size, shape, feel, or function of those devices.
However, safety of implantable medical devices, and compatibility of implantable medical devices with continued patient care, are also a concern. Transponders within implanted medical devices may interfere with the use of certain diagnostic, imaging, or other medical techniques on patients having implants with such transponders. For example, in patients requiring monitoring, examination, and/or screening after implantation of a medical device, it may be necessary for the device to be compatible with the use of various scanning, imaging, and diagnostic techniques, such as magnetic resonance imaging (MM), radiography, ultrasound, tomography, etc. Transponders known in the art may, for example, include ferromagnetic parts, which may interfere with, e.g., an MM performed on a patient having such a transponder in his or her body. Such interference may include, for example, the production of an artifact (e.g., a small imaging void) in imaging results taken of a patient. In such cases, the presence of the artifact in the imaging result may be associated with an increased risk of missing a diagnosis of a patient's condition. For example, a medical professional may miss a diagnosis of recurring cancer due to the artifact obscuring a portion of an MRI showing cancerous cells in the patient. As another example, a rupture in an implant, which may normally be visible on MRI results, may be obscured by an artifact in the results caused by a transponder. As a result, MRI might not be a recommended imaging technique for such a patient, or MRI may need to be combined with another imaging technique such as ultrasound, which may incur additional time and expenses on the part of both the patient and medical professionals. As a further example, transponders of a small size may be difficult for an external reader to read after implants containing those transponders have been implanted in a patient. Alternately, a medical professional may prefer not to use an implant which includes a transponder that would produce unwanted artifacts in imaging results, and/or which may be difficult to read.